ARM: vexpress: create A5 specific board config

This patch creates a new config for the A5 dual core tile that includes the
generic config for the Versatile Express platform.

The generic config has been modified to provide support for the Extended
Memory Map, as used on the A5 core tile.  A5 does not support the legacy
memory map.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@linaro.org>